I am building a NAS (media server and storage ) . One of my big concern is security .I am ready to use any tool LUKS , dm-crypt ,True Crypt . There are a lot of tradeoff .

I use a linux OS (runs on flash ) . The Big Harddrive (ntfs filesystem) will be mounted and used .
I would like to "enable Active Directory Support" . Each user account quota should be minimum and expandable on write .( i dont want to allocate big quota to all users ) .

On security side . Media server uses upnp client to stream and play media .
So those files shouldnot be encrypted . But files on user quota space can be encrypted by user at his own will . He needs a password at the time of access to that particular file .

Can you please address these issues ? any hint or clue on any of these are really appreciated .